(a) Field of the Invention
The present invention relates to Switching Power Supply equipped with current detection circuits both for protection against excess current and current sharing function in parallel operation.
(b) Description of Prior Art
In general, conventional Switching Power Supply for stabilizing D.C. output voltage by switching elements thereof is coupled with excess current protective circuitry for suppressing the secondary excess current of a transformer in a overload condition. On the other hand, in the event of parallel operation of multiple Switching Power Supplies, it is desirable, for reliability's point of view, that a current sharing function may be integrated therewith in order to equalize load current from each Power Supply. For the aforementioned excess current protective circuitry and current sharing function, current detection circuit is indispensable to monitor the secondary output load current of a transformer, which, for example, is disclosed in Japanese Patent Application Laid-open No. 61-231875.
Said Japanese Patent Application discloses such circuit arrangements that between the primary or secondary lines of a transformer is interposed a current detector such as a resistor or a current transformer, and the current flowing therethrough is supplied to a current detection circuit to output current detection signals in proportion to load current thereof.
However, the above conventional art has a few problems described below;
First, in the event of using a resistor as a current detector, power consumption by said resistor itself causes the large loss of said Power Supply itself. Secondly, specifically in the case of Power Supply with a large output current, the current flowing through the resistor also increases, thus such resistor requires larger permissible power capacity, which prevents the saving of space of Power Supply.
On the other hand, in the event of using a current transformer as a current detector, said loss of Power Supply becomes less than using a resistor, however, Power Supply for Super Power needs a large transformer as well as in using said resistor, which prevents the space saving and low-cost manufacturing of Power Supply.